Chapter 14 Flashcards
(33 cards)
what are the three sections of a limb? describe them
stylopod = humerus / femur
zeugopod = radius & ulna / tibia & fibula
autopod = carpals / tarsals
in relation to the limb, describe the three axes (A/P, P/D, D/V)
anterior = thumb
posterior = pinky
proximal = inside (shoulder)
distal = outside (fingers)
dorsal = back of hand (knuckles)
ventral = palm of hand (pads)
what is a limb field?
area on the body before it becomes a limb
what are the three areas of the limb field? describe them
peribrachial = posterior side, under the arm
free limb = what sticks out
shoulder girdle = anterior side, above the arm
describe Harrisons experiment (1918)
did find it, move it, lose it experiments with mesodermal and ectodermal cells of the LIMB FIELD
- mesodermal = (L) stops limb growth
-ectodermal = (L) delayed limb growth bc it needs new skin to form before growing outwards
removed half of the limb field
- by removing any half of the limb field, it can still generate a fully functioning limb
- this occurs naturally thru parasites
describe how parasites alter the limb field in frogs
Parasites enter an animal and can break tissue into groups
- The body forms scar tissue around the parasite, preventing the cells in the limb field from talking to eachother
- Causes multiple limbs to form
*Indicator species -> if something is wrong with a frog, it can indicate something is wrong with everything in the environment
what hox gene is responsible for forming the forelimb bud?
HoxC6
- removing hox causes anteriorization
- adding RA causes posteriorization
describe Fgf8 in limb development
in the apical ectodermal ridge (AER)
- ECTODERMAL CELLS
- helps cells of the limb grow outwards (distally)
describe Fgf10 in limb development
in the mesodermal cells
- helps cells of the limb grow outwards (distally)
what do Tbx5 and Tbx4 transcription factors do in limb development?
Tbx5 = anteriorly, forelimb
Tbx4 = posteriorly, hindlimb
what is the progress zone (PZ)?
cells right under AER, mesenchymal cells
what are the two models for the progress zone? describe them
progress cell model
- the PZ is induced by Fgfs & as these PZ cells grow & divide, they induce specific structures to form
- not all cells remain w/in inducing range
early specification model
- cells decide what they will be prior to dividing
describes Sauder’s grafting experiment (1968)
AER removed = no limb
AER moved = additional limb
AER forelimb & hindlimb swapped = wing in place of leg
AER replaced w/ Fgf bead = normal limb
limb mesenchymal cells replaced = no limb
AER removed in middle of developing limb = shortened limb
what area is Shh located in the limb? what does it specify in limbs?
zone of polarizing activity (ZPA)
- specifies posterior fates
describe the Riddle & Tabin experiment
implanted cells containing Shh-producing virus into anterior portion of limb bud
- produces mirror-image duplication (two pinkies)
determined that Shh & digit identity is dependent on time of exposure (digit 3) and amount of Shh expression (digit 2) –> POLARIZING ACTIVITY
- no Shh = thumb
- lots of Shh = pinky
what did Chiang do?
made mice w/o Shh
- led to dead mice with one eye (cyclopia)
- mice also had improper motor neurons, no A/P limb structure, no spinal column, missing ribs
what did Ohuchi do? (1997)
replaced anterior part of AER w/ Fgf8 = full limb
replaced anterior part of AER w/ Fgf10 = missing some ant. structures of limb
- takes time for Fgf10 to induce Fgf8
- Fgf10 can maintain ZPA, but Fgf8 does it better
*same thing happened when replacing posterior portion!
what molecule does Fgf10 and Fgf8 use to signal and induce eachother?
Wnt
what other Fgf helps keep Fgf8 expressed?
Fgf4
- in the ectoderm
what does Shh do to Fgf’s?
induces them!!
- keeps Fgf10 and Fgf8 active!
what drives the proximal / distal axis in the limb?
Fgf8 (AER) drives distal growth
what drives anterior / posterior axis in the limb?
Shh (ZPA) drives posterior fates
what drives dorsal / ventral axis in the limb?
Wnt7a drives dorsal fates
- engrailed drives ventral fates (blocks Wnt7a)
where is Wnt7a made?
dorsal ectoderm
